1. Field of the Invention
The present invention relates to a combined diversity receiving apparatus which combines two or more antenna receiving signals at the same phase in order to improve the receiving quality in a broadcasting wave where fading or frequency selective multipath distortion exists or in radio communication where fading exists.
2. Description of the Prior Art
FIG. 1 shows a combined diversity receiving apparatus in the prior art disclosed for example in Halpern [IEEE Trans. Commn., Vol. COM-22, No. 8 (1974) pp. 1099-1106]. In FIG. 1, reference numerals 1, 2 designate receiving antennas, numerals 3, 4 first mixers which take the difference frequency between signals from the receiving antennas 1, 2 respectively and output a signal of a feedback circuit 30 as hereinafter described, numerals 5, 6 narrow band-pass filters connected to output of the first mixers 3, 4 respectively, numerals 7, 8 limiters, numerals 9, 10 second mixers which take the difference frequency between output signals of the limiters 7, 8 respectively and output signals from the receiving antennas 1, 2, numeral 11 a coupler which combines the outputs of the second mixers 9, 10, and numeral 30 a feedback circuit which feeds back output signals to the first mixers 3, 4. The feedback circuit 30 has a band-pass filter 12 and a limiter 13, and numeral 14 designates the output signal thereof. The narrow band-pass filters 5, 6 require a band width which allows the random phase component due to fading to pass but the noise and modulation signal component not to pass [refer to for example Miyagaki, Morinaga, Namerikawa: Transaction (B) of the Institute of Electronics and Communication Engineers of Japan, Vol. J63-B No. 1 pp. 9-16 (1980)]. Since the apparatus has structure to form the feedback circuit, a band width as narrow as .+-.2-3 kHz is required in order that the receiving apparatus draw in the system (refer to for example Japanese Patent application laid-open No. 24134/1982).
Operation of the apparatus will now be described. Receiving signals f.sub.c .angle.m(t)+.theta..sub.1, f.sub.c .angle.m(t)+.theta..sub.2 of the receiving antennas 1, 2 are supplied to the first mixers 3, 4 and the second mixers 9, 10. Wherein f.sub.c : carrier freqency, m(t): modulation signal, .theta..sub.1, .theta..sub.2 : random phase due to fading. Then, the output signal 14 of the limiter 13 in the feedback circuit 30 becomes f.sub.o .angle.m(t). f.sub.o represents the output frequency of the feedback circuit 30.
In the first mixers 3, 4, correlation between the above-mentioned received signals and output signals of the limiter 13 is taken. The correlation outputs pass through the narrow band-pass filters 5, 6, which allow the random phase due to fading to pass but eliminate the modulation signal component, and pass through the limiters 7, 8, and then become (f.sub.c -f.sub.o).angle..theta..sub.1, (f.sub.c -f.sub.o).angle..theta..sub.2 respectively. Outputs of the limiters 7, 8 are mixed at the second mixers 9, 10 with receiving signals from the feed forward circuit, and output signals of the second mixers 9, 10 become f.sub.o .angle.m(t) by removing the random phase .theta..sub.1, .theta..sub.2. The output signals are combined by the coupler 11, and then the composite signal passes through the band-pass filter 12 and the limiter 13 and is fed back to the first mixers 3, 4.
Thus the combined diversity receiving apparatus in the prior art is constituted to remove the random phase due to fading. In the broadcasting wave, however, in addition to the random phase due to fading, frequency selective multipath distortion which distorts amplitude and phase of the modulation signal component causes a problem. In the above-mentioned apparatus in the prior art, no steps were taken with respect to the frequency selective multipath distortion. Consequently, although the multipath distortion may be removed by chance in the receiving wave of a specific level, the multipath distortion cannot be reduced in the usual case; for example, when the receiving antennas 1, 2 are supplied with an FM wave without multipath distortion and an FM wave having multipath distortion in similar degree.
The reason why the frequency selective multipath distortion is not reduced in the above-mentioned apparatus of the prior art will now be described specifically.
FIG. 2(a) serves the spectrum of an FM wave modulated by sinusoidal modulation signals without multipath. In FIG. 2(a), f.sub.c represents the carrier frequency, and f.sub.m represents the frequency of the modulation signal. Numeral 15 designates the spectrum of carrier, numeral 16a the upper side wave of the fundamental wave of the modulation signal, numeral 16b the lower side wave thereof, numeral 17a the upper side wave of the second harmonic wave of the modulation signal, and numeral 17b the lower side wave thereof. FIG. 2(b) shows a vector diagram of the FM wave in connection with FIG. 2(a). For simplification, only the fundamental wave of the modulation signal is studied but the generality is not lost. When the FM wave shown in FIG. 2(a) is inputted to the receiving antennas 1, 2, the spectrum of the output 14 of the feedback circuit 30 also becomes similar to that of FIG. 2(a). However, the carrier frequency f.sub.c is converted into the output frequency f.sub.o, and this is shown in FIG. 2( c).
Next, assume that the FM wave with frequency selective multipath is inputted to the receiving antenna 2. The spectrum of the FM wave in this case is shown in FIG. 2(d), and a vector diagram thereof is shown in FIG. 2(e). Then correlation between the received signals and the output 14 of the feedback circuit 30 is achieved by the first mixers 3, 4. A vector diagram and a spectrum in the output of the first mixer 3 in a system without multipath are shown in FIG. 2(f) and FIG. 2(g) respectively. A vector diagram in the output of the first mixer 4 in system having multipath is shown in FIG. 2(h). The spectrum in this case is shown in FIG. 2(i). In FIG. 2(i), numeral 18 designates the spectrum of (f.sub.c -f.sub.o), numeral 19a the spectrum of the multipath distortion component of the upper side band, and numeral 19b the spectrum of the multipath distortion component of the lower side band.
Since the band width of the narrow band-pass filter 6 is such as to eliminate the modulation signal component, the distortion components 19a, 19b in FIG. 2(i) do not pass. Consequently, output signals in the second mixers 9, 10 become the same as the receiving input signals and the distortion components are not removed. Output signals of the second mixers 9, 10 are combined in the coupler 11, and its output vector is shown in FIG. 2(j). It is clear that the multipath reduction effect does not exist.
Another combined diversity receiving apparatus in the prior art is disclosed in Japanese Patent application laid-open No. 125536/1982, as shown in FIG. 3. In FIG. 3, parts identical or corresponding to those in FIG. 1 are designated by the same reference numerals. Numerals 20, 21 designate amplifiers which are connected between the band-pass filter 5 and the second mixer 9 and between the band-pass filter 6 and the second mixer 10 respectively. Numeral 22 designates a detector.
Operation of the apparatus will now be described. Receiving signals f.sub.c .angle.m(t)+.theta..sub.1, f.sub.c .angle.m(t)+.theta..sub.2 of the receiving antennas 1, 2 are mixed at the first mixers 3, 4 with output signal f.sub.o .angle.m(t) of the feedback circuit 30. The mixed signals pass through the band-pass filters 5, 6 and the amplifiers 20, 21, and the output signals become (f.sub.c -f.sub.o).angle..theta..sub.1, (f.sub.c -f.sub.o).angle..theta..sub.2 respectively. The output signals and the receiving signals are mixed at the second mixers 9, 10. Output signals of the second mixers 9, 10 are combined by the coupler 11, and the composite signal passes through the band-pass filter 12 and the limiter 13 and is fed back to the first mixers 3, 4. The output signal 14 is detected by the detector 22.
In the combined receiving apparatus constituted as in FIG. 3, difference is produced between two feed forward loops 40, 41 in the fixed phase shift, resulting in distortion or noise in the output signal of the detector 22.
The cause of above-mentioned problems in the prior art will be described. Assume that the group delay time of the band-pass filters 5, 6 is .tau..sub.1, .tau..sub.1 ' and the delay time of the feedback circuit 30 is .tau..sub.2. Also assume that the phases of the receiving signals of the receiving antennas 1, 2 are EQU 2.pi.f.sub.c t+.theta..sub.1 EQU 2.pi.f.sub.c t+.theta..sub.2
respectively, and the phases of the output signal 14 of the feedback circuit 30 are 2.pi.f.sub.o t. In the first mixers 3, 4, the receiving signals and the output signal 14 of the feedback circuit 30 are mixed, and the phases of the mixed output signals become EQU 2.pi.(f.sub.c -f.sub.o)t+.theta..sub.1 EQU 2.pi.(f.sub.c -f.sub.o)t+.theta..sub.2
respectively. The output signals of the first mixers 3, 4 pass through the band-pass filters 5, 6, and the output phases become ##EQU1## respectively. f.sub.IF represents the center frequency of the band-pass filters 5, 6. Signals passing through the band-pass filters 5, 6 are mixed at the second mixers 9, 10 with the received signals, and the phases of the output signals become EQU 2.pi.f.sub.o t+2.pi.f.sub.IF .tau..sub.1 ( 1) EQU 2.pi.f.sub.o t+2.pi.f.sub.IF .tau..sub.1 ' (2)
respectively.
Signals having phases of formulae (1), (2) are combined by the coupler 11. If .tau..sub.1 =.tau..sub.1 ', the received signals are combined at the same phase. However, since .tau..sub.1 and .tau..sub.1 ' do not become completely equal in the actual state, a phase difference is produced between the output signals of the second mixers 9, 10. Consequently, the phase difference may cause distortion or noise in the detector output.
According to analysis of Halpern [IEEE Trans. Commun., Vol. COM-22, No. 8 (1974) pp. 1099-1106], the frequency of the feed forward loop is represented by ##EQU2## Wherein k is an integer so that f.sub.IF is drawn within the band width of the band-pass filters 5, 6. Consequently, if a difference exists between .tau..sub.1 and .tau..sub.1 ', following state will occur. When the receiving antenna 1 mainly acts as EQU f.sub.IF =(f.sub.c .tau..sub.2 +k)/(.tau..sub.1 +.tau..sub.2)
frequency f.sub.o of the output signal 14 becomes ##EQU3## Then the feed forward loop 41 of the receiving antenna 2 also acts at the frequency f.sub.IF. When the level of the receiving antenna 1 is lowered and the receiving antenna 2 mainly acts as EQU f.sub.IF '=(f.sub.c .tau..sub.2 +k)/(.tau..sub.1 '+.tau..sub.2)
the frequency of the output signal 14 becomes EQU f.sub.o '=f.sub.c -f.sub.IF '
Consequently, the frequency of the output signal 14 jumps from f.sub.o to f.sub.o '.
The phase shift of the feed forward loop occurs also due to components other than the band-pass filters 5, 6. Consequently, even if .tau..sub.1 =.tau..sub.1 ', the phase shift difference between the feed forward loops may occur.
In the combined diversity receiving apparatus of the prior art, output signal components from the second mixers 9, 10 are as shown in FIG. 4. In FIG. 4, f.sub.IF represents the center frequency of the first band-pass filters 5, 6, f.sub.o represents frequency of output signal (desired signal) from the feedback circuit, and W represents the transmission band width of the received input signal. Assuming that the delay time of the first band-pass filters 5, 6 is .tau..sub.1, and the delay time and the band width of the second band-pass filter 12 is .tau..sub.2, B respectively, the diversity system can be operated (i.e., the diversity effect is obtained) if following condition is satisfied. EQU .tau..sub.3 &gt;&gt;.tau..sub.2 and W&lt;&lt;B (1)
Consequently, if value of .tau..sub.2 is small, i.e., if value of B is large, formula (3) will be satisfied. In this case, however, spurious (unrequired wave) components exist in the band width B of the second band-pass filter 12 as shown in FIG. 4 and problems may occur as hereinafter described.
If the second band-pass filter 12 allows the f.sub.IF component to pass, the f.sub.IF component passes through the first mixers 3, 4 and then becomes the center frequency of the band-pass filters 5, 6. The components of (f.sub.c +f.sub.IF) is mixed at the first mixers 3, 4 with the received input signals and becomes the f.sub.IF component. In any of these cases, the spurious component exists and therefore the diversity effect is not obtained well.
Furthermore, in the combined diversity apparatus of the prior art, the band-pass filter 12 of the feedback circuit 30 is set to a band width such that the spurious component is removed [Suwa, Hattori: Technical Report of the Institute of Electronics and Communication Engineers of Japan, CS81-55 pp. 91-96 (1981)].
If the combined diversity receiving apparatus of the prior art is used in the receiving of a the broadcasting of wide transmission band such as FM broadcasting, problems may occur as hereinafter described. Assuming that the delay time of the band-pass filters 5, 6 is .tau..sub.1, and the delay time of the band-pass filter 12 of the feedback circuit 30 is .tau..sub.2, this system must satisfy the condition .tau..sub.1 &gt;&gt;.tau..sub.2 according to analysis of Halpern (refer to above-mentioned reference). However, if .tau..sub.2 is made small so as to satisfy .tau..sub.1 &gt;&gt;.tau..sub.2, the band width becomes wide and the band-pass filter 12 allows not only the desired signal but also the spurious component to pass. In this state, if the receiving level is lowered, the desired signal level of the second mixers 9, 10 is also lowered, and therefore the desired signal level passing through the band-pass filter 12 becomes lower than the level of the spurious component. Consequently, the limiter 13 does not amplify the desired signal and this system does not act properly.